The video display systems of portable electronic equipment and computer data terminals commonly employ liquid-crystal displays. Liquid-crystal displays often require some form of backlighting in order for the video display to have functional brightness and visibility. The backlighting of liquid-crystal displays frequently implement electroluminescent panels to provide the necessary light intensity. Electroluminescent backlight panels are composed of an electroluminescent material that emits visible light when driven with an alternating current.
In portable battery powered electronic equipment and computer data terminals, conservation of battery energy is an important design criterion. Electroluminescent backlight panels operate at high voltages and consume a large portion of the onboard available battery power. A unique feature of the material of which the electroluminescent backlight panel consists is that it exhibits capacitive behavior, meaning it is capable of holding an electric charge, and therefore some of the energy supplied to the electroluminescent backlight is stored and unused as a result of that capacitance. Thus a concomitant feature of the electroluminescent panel is that it essentially functions as a large capacitor. The unused energy stored capacitively in the electroluminescent backlight panel may be recovered and restored back into the battery to significantly increase the duration of the battery charge.
Drive circuits for electroluminescent panels are known in the prior art which often employ transformers or other inductive devices in conjunction with the inherent capacitance of the electroluminescent panels. Kindlmann, U.S. Pat. No. 4,527,096, discloses a circuit in which an electroluminescent lamp in combination with several inductors is progressively charged with successive high frequency current pulses provided by a converter to thereby increase the efficiency and to enhance the light output from the electroluminescent lamp. Other methods for driving electroluminescent devices also exist in the prior art. Often the inductance of the transformer, typically an autotransformer, is configured in combination with the inherent capacitance of an electroluminescent device to resonate at the desired operating frequency at which the electroluminescent device operates in order to improve power efficiency. Koenck, U.S. Pat. No. 5,345,146, describes a drive circuit for an electroluminescent panel with reduced physical transformer size, reduced current draw from the power supply battery, and accommodations for the variations in the inherent capacitance and inherent resistance of the electroluminescent devices. Koenck further describes the use of energy stored in an inductor/capacitor resonant tank circuit for driving the electroluminescent device in a subsequent charge cycle. However, the prior art does not anticipate recovering the unused energy to be restored back into the battery.